Sensors for measuring strong forces are well known in the art. For example, sensors to identify strong mechanical, acoustic, magnetic, electrostatic, and thermal forces are well known. There is a growing demand for fast, sensitive, inexpensive, and reliable sensors to identify micro-forces. As used herein, the term micro-force refers to a force that is capable of deflecting, but not destroying, a micron scale cantilever. The micro-forces may be in the form of chemical, mechanical, thermal (through conduction, convection or radiation), acoustic, magnetic, or electrostatic forces or combinations of these forces. By way of illustration, the invention is described in connection with the sensing of micro-forces arising from biological interactions, which create micro-forces in the form of chemical-mechanical forces.
In the context of measuring micro-forces associated with biological interactions, one could focus, for example, on the need to detect disease and biological pathogens. As discussed below, conventional methods for biological sensing suffer from at least one of several problems, including long analysis time, high instrumentation cost, lack of sensitivity, and the inability for real-time monitoring.
Immunosensors utilize the specificity of antibody-antigen (Ab-Ag) interactions in combination with a variety of transduction techniques. Electrochemical devices monitor the current at a fixed potential (amperometry) or voltage at zero current (potentiometry), or conductivity or impedance changes due to biochemical reaction. Optical methods use the effect of biological events on light absorption, fluorescence, refractive index variation, or other optical parameters. Techniques such as surface plasmon resonance (SPR) have shown promise in providing direct measurement of Ag-Ab interactions occurring at the surface-solution interface. The major draw back of optical systems is the use of complex optical components and their high cost. Thermometric devices operate by measuring enthalpy changes during the biological reaction. Piezoelectric devices utilize surface acoustic waves to detect changes in resonance in the presence of Ag-Ab reactions. The principal attraction of piezoelectric immunosensors is their ability to directly monitor the binding of Ab-Ag reactions encountered in affinity sensing. The enzyme linked immunosorbent assay (ELISA) is a sensitive technique for diagnosis. Enzyme immunoassays combine the specific recognition of antibodies for their target molecules with the catalytic power of enzymes into a single sensitive and relatively simple test. Antibodies, bacterial and viral antigens, nucleic acids, and many diverse molecules are detected by an indicator system in which the bound enzyme convert a colorless chromogenic substrate into brightly colored products. ELISA, however, involves multiple steps making it labor intensive.
Micron scale cantilever beams are currently used as ultra-sensitive force sensors in many different applications. Cantilevers can be fabricated in arrays using almost any material that is compatible with microfabrication. For example, semiconductor-based cantilevers are used in atomic force microscopes (AFMs). The length, width, thickness, and modulus of a cantilever beam can be controlled to produce spring constants, g, between 0.01–10 N/m and with resonant frequency in the range of 10–500 kHz. Cantilever deflections, d, can be optically detected with resolutions of about 1 Å or 100 pm, which leads to a force resolution, F=gd, in the range of 1–100 pN.
In addition to their wide use in AFMs where the force is applied at a single point (the tip), microcantilevers have recently been used as sensors for measuring extremely small bending moments that are produced by thermally or chemically generated stresses over the whole cantilever surface. It has been demonstrated that a cantilever beam can be used as a calorimeter to detect the heat of a catalytic reaction. Cantilevers have also been used as infrared sensors where the thermal stress is produced by infrared absorption. Each of these systems relies upon complex and expensive optical processing equipment.
Microcantilevers have also been used for detecting enzyme-mediated catalytic biological reactions with femtoJoule resolution, as shown in T. Thundat, et al., “Microcantilever Sensors”, Microscale Thermophysical Engr. 1, 185–199 (197). Thundat and co-workers disclose an antibody-Antigen (Ab-Ag) reaction using a cantilever sensor, presumably due to surface stresses generated by intermolecular force interactions of the Ag-Ab complex. Thundat analyzes frequency changes in a single resonating cantilever beam. Thus, the Thundat system requires active circuitry to operate the cantilever beam and to measure cantilever beam frequency responses to biological reactions.
In view of the foregoing, it would be highly desirable to provide an improved technique for sensing micro-forces. In particular, it would be highly desirable to provide a technique with reduced analysis time, low instrumentation cost, high sensitivity, and real-time monitoring.